Nuclear power reactors are well known and are discussed, for example, by M. M. El-Wakil in "Nuclear Power Engineering", McGraw-Hill Book Company, Inc., 1962.
In a known type of nuclear power reactor, for example, as used in the Dresden Nuclear Power Station near Chicago, Ill., the reactor core is of the heterogenous type. In such reactors the nuclear fuel comprises elongated rods formed of sealed cladding tubes of suitable material, such as a zirconium alloy, containing uranium oxide and/or plutonium oxide as the nuclear fuel, for example, as shown in U.S. Pat. No. 3,365,371. A number of such fuel rods are grouped together and contained in an open-ended tubular flow channel to form a separately removable fuel assembly or bundle. A sufficient number of fuel assemblies are arranged in a matrix, approximating a right circular cylinder, to form the nuclear reactor core capable of self-sustained fission reaction. The core is submerged in a fluid, such as light water, which serves both as a coolant and as a neutron moderator.
A well-known and widely used type of fuel assembly is shown by C. R. Mefford et al in U.S. Pat. No. 3,697,376. Such a fuel assembly is formed by an array of spaced fuel rods supported between upper and lower tie plates, the rods being several feet in length, on the order of one-half inch in diameter and spaced from one another by a fraction of an inch.
As shown in U.S. Pat. No. 3,697,376, the lower tie plate is formed integrally with a tapered nose piece or nozzle which engages a fuel assembly support socket of the core support structure of the nuclear reactor. The nose piece is formed with openings for receiving pressurized fluid coolant and directing it upward past the fuel rods.
To contain the coolant flow, the fuel assembly is surrounded by an open ended coolant flow channel. The lower open end of the flow channel is an unrestrained slip fit over the lower tie plate/nozzle which results in the problem of control of coolant leakage addressed in U.S. Pat. No. 3,697,376.
Other fuel assembly arrangements are known, such as shown by Suvanto et al in U.S. Pat. No. 3,697,375, wherein the lower tie plate and the nozzle are formed as separate pieces and the lower end of the flow channel is permanently attached to the nozzle. This arrangement solves the coolant leakage problem and can provide other advantages as discussed in U.S. Pat. No. 3,697,375.
The flow channel typically is formed of a zirconium alloy (to minimize neutron absorption) while the nozzle typically is formed of stainless steel. Thus welding of the flow channel to the nozzle as a method of attachment is not practical.
Therefore, attachment of the flow channel to the nozzle has been accomplished by the use of rivets or screws as shown and described in U.S. Pat. No. 3,697,375. However, such attachment by rivets or screws has not been entirely satisfactory. This is because differential thermal expansion of the parts can result in over-stressing and consequent failure (or at least loosening) of the rivets or screws.
Furthermore, if the flow channel is firmly attached to the nozzle (as with screws or rivets), the greater expansion and contraction of the stainless steel nozzle with changes in temperatures, as compared to the zirconium alloy flow channel, can cause bending and stressing of the lower end of the flow channel.
Therefore, an object of the invention is a channel-to-nozzle attachment which substantially avoids stressing of the parts with differential thermal expansion.